Three dimensional atom probe investigation of vanadium nitride precipitates and the role of oxygen and boron in rapidly solidified 316 stainless steel

A 316 stainless steel that was rapid solidification processed (RSP) by gas atomization and hot extrusion of the powder has been studied previously by conventional atom probe field ion microscopy (APFIM). Vanadium, nitrogen, and oxygen have been introduced intentionally as alloying elements for the purpose of improving the mechanical properties through fine-scale precipitation. Previous efforts to locate the oxygen in the structure using APFIM were inconclusive, largely due to poor counting statistics. An energy-compensated three-dimensional position-sensitive atom probe (3DAP) has been used in the present study to search for oxygen in cavities, precipitates, and grain boundaries in this alloy. As a direct result of the much greater number of atoms detected and the three-dimensional imaging, oxygen concentrations on the order of 1 atomic% have been found both inside nitride precipitates and in the vicinity of grain boundaries. Boron was also found unexpectedly in concentrations of up to 2 atomic% inside nitride precipitates and in the vicinity of grain boundaries. These findings are important to developing an understanding of the grain growth resistance and precipitate stability both in this alloy and in this processing method in general.